First and deadliest of all is a founder’s unwavering belief that he or she understands who the customers will be, what they need, and how to sell it to them. Any dispassionate observer would recognize that on Day One, a start-up has no customers, and unless the founder is a true domain expert, he or she can only guess about the customer, problem, and business model. On Day One, a start-up is a faith-based initiative built on guesses.

To succeed, founders need to turn these guesses into facts as soon as possible by getting out of the building, asking customers if the hypotheses are correct, and quickly changing those that are wrong."